Greenhouse effect: Different components

Greenhouse effect: Different components

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Greenhouse gases make up a tiny fraction of the atmosphere, but have a large effect on the climate. The different greenhouse gases trap varying amounts of outgoing infrared heat energy, depending on their quantity and characteristics. Together with clouds, these gases make up the components of the greenhouse effect.

Water vapour is by far the most abundant greenhouse gas, and scientists estimate it accounts for about half of the total heat-trapping ability of the greenhouse effect. But water vapour is also a very short-lived greenhouse gas, being removed from the atmosphere when it condenses into clouds and falls to the ground as rain. So, unlike other greenhouse gases such as carbon dioxide, water vapour doesn’t accumulate in the atmosphere, because its quantity is directly controlled by the prevailing weather conditions. This means that water vapour can neither strengthen nor weaken the greenhouse effect by itself, though it acts as an amplifier – or positive feedback – following changes in other greenhouse gases.

As well as being present in the atmosphere as a gas, water vapour also condenses into liquid water on tiny particles of dust or ice – forming water droplets and clouds. Clouds are an important absorber of outgoing
infrared energy. Scientists estimate that clouds account for about a quarter of the total heat trapped by the greenhouse effect. But clouds also reflect incoming sunlight, so they have both warming and cooling effects on the climate, depending on the type of cloud. Low-level clouds tend to reflect more energy than they trap, resulting in an overall cooling effect. In contrast, high-level wispy clouds tend to trap more energy than they reflect.

Each carbon dioxide (CO2) molecule absorbs infrared energy more strongly than each water vapour molecule. But there’s less CO2 than water vapour in the atmosphere, so CO2 makes a smaller overall contribution to the greenhouse effect. Scientists estimate that CO2 accounts for about a fifth of the total heat trapped by the greenhouse effect. CO2 is absorbed by oceans and plants, but doesn’t get caught up in rain and weather systems. So CO2 can accumulate in the atmosphere over years and decades, meaning its quantity can increase and decrease independently of the climate, strengthening or weakening the greenhouse effect. On average, CO2 can stay in the atmosphere for over 100 years.

Apart from small quantities of water vapour and carbon dioxide, there are also even smaller quantities of other greenhouse gases in the atmosphere, including methane, ozone and nitrous oxide. Methane and nitrous oxide molecules are strong absorbers of infrared energy. Nitrous oxide can stay in the atmosphere for over 100 years – similar to the lifetime of carbon dioxide – while methane has a shorter lifetime of about ten years. Ozone also acts as a greenhouse gas, but it absorbs ultraviolet sunlight – mostly in the stratosphere – as well as infrared heat energy. Together, these and tiny quantities of other gases account for about 5% of the total heat-trapping ability the greenhouse effect.

The contribution of greenhouse gases and clouds to the greenhouse effect depends on how strongly they absorb infrared energy, their quantity and how long they remain in the atmosphere. In addition, just as visible light comes in all the colours of the rainbow, infrared energy comes in different ‘shades’ – frequencies. Different components (greenhouse gases and clouds) absorb different frequencies. But at some frequencies all the available infrared energy is absorbed by just some of the components, so the rest of the gases or clouds are redundant. These different factors often have competing effects, which scientists calculate when estimating the overall contribution of each component of the greenhouse effect.

Water vapour responds rapidly to weather conditions such as temperature and air pressure, and can condense into clouds and rain. In contrast, other greenhouse gases such as carbon dioxide and methane don’t respond directly to weather conditions and can stay in the atmosphere for years, decades or longer. These ‘long-lived’ greenhouse gases are also well mixed in the lower atmosphere, meaning that they’re spread out more or less evenly around the globe. In contrast, water vapour concentrations in the lower atmosphere vary widely – from less than 1% in dry areas to as much as 4% in humid areas.

Sometimes the best place to look for answers is where you’d least expect them. Ingo would agree – he tries to find out how Earth’s atmosphere works by studying other planets. ‘I use spacecraft observations to measure atmospheric gas properties at other planets,’ he says. Ingo also uses supercomputers to model extraterrestrial atmospheres. Understanding the behaviour of gases on other planets helps scientists to understand what goes on in the Earth’s atmosphere. ‘The most exciting discoveries I’ve been involved in relate to the atmosphere of Saturn’s largest moon, Titan, a body resembling Earth in many ways, but yet so different.’

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There are many institutions and organisations around the world researching climate science, how our climate is changing, and ways of responding. Here are just a few…

British Antarctic Survey (BAS)

Department for the Environment, Food and Rural Affairs (Defra)

Department of Energy and Climate Change (DECC)

Energy Saving Trust (EST)

Environmental Change Institute (ECI)

European Space Agency (ESA)

The Geological Society (GS)

Grantham Institute for Climate Change (GICC)

Intergovernmental Panel on Climate Change (IPCC)

Met Office (MO)

National Academy of Sciences (NAS)

National Aeronautics and Space Administration (NASA)

National Oceanic and Atmospheric Administration (NOAA)

National Oceanography Centre (NOC)

The Royal Society (RS)

Tyndall Centre for Climate Change Research (TCCCR)

UK Climate Impacts Programme (UKCIP)

United Nations Framework Convention on Climate Change (UNFCCC)

World Climate Research Programme (WCRP)

World Meteorological Organization (WMO)

Climate

A summary of the weather in a particular region over a period of at least ten years, but more commonly defined over 20 - 30 years. The climate describes both the average weather conditions (for example temperature, rain, snow and wind) in a particular region as well as the extremes.

Infrared energy

Greenhouse effect

A natural process in the atmosphere whereby greenhouse gases trap heat near the Earth’s surface. This keeps the Earth’s surface temperature roughly 30 °C warmer than it would otherwise be. Changing the amount of greenhouse gases in the atmosphere changes the strength of the greenhouse effect, which in turn changes the temperature of the atmosphere near the Earth’s surface.

Water vapour

Water in the form of a gas. Water vapour is the most common greenhouse gas.

Carbon dioxide

An important greenhouse gas, with the chemical formula CO2. After water vapour, carbon dioxide is the biggest contributor to the greenhouse effect.

Feedback

A process with two interconnected factors whereby the first factor affects the second, and the change in the second factor in turn affects the first. This can either increase the rate of the process (positive feedback) or decrease it (negative feedback).

An example of a positive feedback process is the melting of ice floating on an ocean. When the local temperature increases it melts the ice and reveals the dark ocean underneath. The ocean absorbs more of the Sun’s energy than the ice, causing the local temperature to increase further and leading to even more melting.

Methane

A greenhouse gas about 25 times more powerful than carbon dioxide. Methane contains carbon and is one of the gases exchanged in the carbon cycle.

Ozone

A powerful greenhouse gas found in the atmosphere. Ozone is most abundant in the stratosphere, where it absorbs harmful ultraviolet energy from the Sun.

Nitrous Oxide

A greenhouse gas 298 times more powerful than carbon dioxide.

Molecules

Particles that form the building blocks of solids, liquids and gases. Molecules are made of even smaller particles called atoms.

Visible light

Energy of a specific frequency range that can be detected by the human eye.